The invention relates to a valve for controlling fluids. From European Patent Disclosure EP 0 477 400 A1, a valve is already known which is actuatable via a piezoelectric actuator. This known valve has an arrangement for an adaptive mechanical tolerance compensation, effective in the stroke direction, for a travel transformer of the piezelectric actuator, in which the deflection of the piezoelectric actuator is transmitted via a hydraulic chamber.
The hydraulic chamber, which functions as a so-called hydraulic step-up means, encloses a common compensation volume between two pistons defining this chamber, of which one piston is embodied with a smaller diameter and is connected to a valve member to be triggered, and the other piston is embodied with a larger diameter and is connected to the piezoelectric actuator. The hydraulic chamber is fastened between the two pistons in such a way that the actuating piston of the valve member, which piston is retained in its position of repose by means of one or more springs relative to a predetermined position, executes a stroke that is increased by the step-up ratio of the piston diameter when the larger piston is moved a certain travel distance by the piezoelectric actuator. The valve member, piston and piezoelectric actuator are located one after the other on a common axis.
Via the compensation volume of the hydraulic chamber, tolerances caused by temperature gradients in the component or different coefficients of thermal expansion of the materials used and possible settling effects can be compensated for without causing a change in position of the valve member to be triggered.
Compensating for changes in length of the piezoelectric actuator, the valve member or the valve housing by means of the hydraulic chamber disposed between two pistons requires a complicated construction, however, and is problematic in terms of the incident leakage losses and the refilling of the hydraulic chamber.
Furthermore, it is well known for a piezoelectric actuator to be constructed of a plurality of thin layers, in order to attain the longest possible stroke. So that these layers will not separate from one another when current is supplied to the piezoelectric actuator, the piezoelectric actuator must be prestressed, and the force to be brought to bear can amount to approximately 1000 N.
In practice, for prestressing the piezoelectric actuator, either cup springs or flat spiral springs are used. A disadvantage of this is that the springs required for the prestressing dictate a complicated design and require a large amount of space, and the latter leads to a correspondingly large diameter of the entire valve, thus limiting the options for installing the valve. Using springs as prestressing elements also has the disadvantage that over the duration of their use, they cause friction rust.
The object of the invention is to create a valve for controlling fluids in which the prestressing of a piezoelectric actuator and tolerance compensation are achieved while requiring little installation space, with a simple structure having as few components as possible.
The valve for controlling fluids according to the invention has the advantage that with the hydraulic chamber embodied as a hydraulic spring, a prestressing element for the piezoelectric actuator and a compensation element, in particular for temperature-dictated elongation tolerances, are simultaneously achieved.
In the hydraulic prestressing element according to the invention with integrated tolerance compensation for the piezoelectric actuator, the prestressing is achieved hydraulically and with little demand for space; by the omission of springs or other mechanical prestressing elements, a desirable slender shape of the entire valve is possible.
Because of the reduction in the number of components required to act as both a prestressing element and a tolerance compensating element, which according to the invention is integrated with the prestressing element, the production costs and effort of assembly can be reduced markedly.
A significant advantage of the invention is furthermore that by dimensioning of the hydraulic chamber, the hydraulic spring, and the piston that plunges into the spring and the chamber, the overall rigidity of the system can be enhanced. Since the rigidity of the hydraulic spring is dependent on the cross-sectional area of the piston, for the same pressure the rigidity of the hydraulic spring and thus the prestressing force on the piezoelectric actuator can be increased, if the cross-sectional area of the piston plunging into the hydraulic chamber is enlarged accordingly. In the static case, even at a high spring rate, a disadvantageous change in length of the entire device can be averted, if the piezoelectric actuator, valve member or valve body changes its length, for instance on heating up. In addition, upon a dynamic actuation, the rigidity of the hydraulic spring on which the piston is braced becomes greater, the greater the selected diameter of the piston. This has the further advantage that the stroke losses of the piston decrease as the diameter increases.
Further advantages and advantageous features of the subject of the invention can be learned from the is specification, drawing and claims.